Interferometric gauge



April 5, 1949. G. E. MERRI TT 2,466,322

INTERFEROMETRIC GAUGE Filed Dec. 16, 1944 5 Sheets-Sheet l lzvl mvrolz' WITNESS: W Z. Herr/d0 .zww w/Vwu, BY

April 5, 1949. G. E. MERRlTT INTERFEROMETRIC GAUGE Filed Dec. 16, 1944 5 Sheets-Sheet 3 INVENTOR.

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A ril 5, 1949. e. E. MERRITT INTERFEROMETRIC GAUGE 5 Sheets-Sheet 4 Filed Dec. 16 1944 INVENTOB year/ye fi mermltf/ BY ATT NEY WITNESS: 40w WW April 5, 1949.

G. E. MERRITT INTERFEROMETRIC GAUGE 5 Sheets-Sheet 5 Filed Dec. 16, 1944 INVEN'TOR. georye E. file/11w TORNEY wmvzss; 5% Id.

Patented Apr. 5, 1949 INTERFEROMETRIC GAUGE George E. Merritt, Elmira Heights, N. Y., assignor to Bendix Aviation Corporation, South Bend, Ind., a corporation of Delaware Application December 16, 1944, Serial No. 568,517

13 Claims. 1

The present invention relates to an interfere metric gauge, and more particularly to an instrument and a procedure for utilizing 'said instrument for accurately comparing the internal dimensions of hollow articles. The method of manufacturing articles to precise dimensions utilizing the gauge herein disclosed is claimed in the co-pending application Serial 611,456 filed August 20, 1945.

In the manufacture of interengaging parts requiring very accurate fits such for instance as in the manufacture of fuel pumps for internal combustion engines, it has so far been impossible to secure general interchangeability of parts due largely to the lack of sufliciently stable and sensitive gauging means, and of a sufliciently accurate control of the machine operations by virtue thereof. It has been found necessary, therefore, in the assembly of such parts, to maintain supplies of parts graded by small steps of variation in the critical dimension, and assemble the parts selectively to secure the desired fit. Thus in the manufacture of plungers and bushings for fuelpumps where the tolerance in fit is for example plus or minus 5 millionths of an inch, it has heretofore been found necessary to maintain eight classifications of parts as to diametral measurement, and assemble the parts by proper selection from such classes. The method is obviously disadvantageous as compared to completeinterchangeability of parts.

It is an object of the present invention to provide a novel interferometric internal gaugewhich is suificiently accurate and sensitive to indicate variations in dimensions in the order of millionths of an inch.

Another object is the provision of such a device which is particularly adapted for the measurement of thevinternal diameters of cylinders, and which provides highly accurate means for obtaining perpendicularity ofthe measurement of the distance between two parallel optically flat surfaces, especially in the direction at right angles to the axis of the cylinder.

Another object is the provision of such a device which provides highly accurate means for adjusting the axis of the cylinder relative to the measuring device in a line parallel with the surfaces of the optical flats.

Another object is the provision of such a device for contacting the work, and means are provided a z which provides highly accuratemeans for setting the gauging points in the cylinder to be measured exactly on the diameter of the cylinder and of maintaining the perpendicularity of the line intersecting said points with theaxis of the cylinder and with the planes of the optical flats.

It is another object to provide such a device embodying a pair of caliper arms, each of which is rigidly fixed to one of a pair of optical flats which are hingedly connected to a common base.

It is a further object to provide such a device having means for producing and observing light patterns formed by interference between the light reflected from the adjacent surfaces of the flats.

It is another object to provide such a device in which the means for observing the light-pattern comprises. an optical system'whichis mounted independently of the gauge mechanism including the optical flats, and which is universally adjustable and extensible.

It is another object to provide such a' device in i which said arms are readily and accurately adjustable with respect to the flats, both laterally and vertically.

It is another object to provide such a device in which the caliper arms are provided with jewels for accurately controlling the pressure. of [the jewels against the work.

It is another objectto provide such a device in ,which the hinge connection is free from'lost motion or abaxial deflection, andsubstantialLv free from static friction. i n 1 It is another object to provide such a device having rigid unitary holders for the flats.

It is another object to provide such adevice in which the hinge connection is provided by flexible plates which are rigidly anchored to the holders for the optical flats.

It is another object to provide a novel method of manufacturing parts with very small dimen sional tolerances so as to secure interchangeability of parts which require close flts.

It is another object to provide such a I which utilizes a gauge of the above type to, accomplish the desired dimensional accuracy.

Further objects and advantages will be apparent from the following description taken in connection with the accompanying drawings in which:

method I Fig. 1 is a perspective view of the optical system and gauge constituting a preferred form of the invention shown in perspective, the electrical system for the light source being shown diagrammatically;

Fig. 2 is a diagrammatic showing of the optical system of the gauge;

Fig. 3 is a side elevation on an enlarged scale, partly broken away, of the gauge mechanism per se;

Fig. 4 is a front view of the structure illustrated in Fig. 3;

Fig. 5 is a top view of the gauge mechanism;

Fig. 6 is a detail partly in section showing the method of anchoring the hinge members for the optical flats;

Fig. '7 is a detail in perspective of the holder for the upper optical flat with its gauge arm detached therefrom;

Fig. 8 is a detail partly in section showing the light source and the viewing lens system;

Fig. 9 is a detail in perspective of the holder for the light source;

Fig. 10 is a sectional detail on an enlarged scale of the lens and mirror system for directing the light onto the gauge and for receiving the reflected light and directing it to the observing lens system; and

Fig. 11 is a detail in perspective of the base or stage for the gauge, the work and the standard gauge fixture, the latter being shown partly disassembled for the purpose of clarity.

In Fig. 1 of the drawing there is illustrated a light source l which is here shown in the form of a gaseous tube arranged to be activated by a source of electricity at high potential such as a transformer 2 the secondary of which is connected by leads 3 and I to the tube I, and the primary of which is energized from a suitable power line through controlling resistors 5,'6 and a manual switch I.

The light source I is mountedin a holder 8 best shown in Fig. 9, which holder is provided with a small opening 9 in line with the central portion of the tube The holder 8 is mounted in the end of a cylinder II which is adjustably supported in a bracket l2 carried by a cross bar l3 forming part of a suitable supporting frame not illustrated. It will be understood that the entire instrument is preferably housed in an airconditioned and temperature controlled chamber such as illustrated for instance in the copending application of Merritt, Serial Number 549,709, filed'August 16, 1944 now Patent No. 2,450,839, issued Oct. 5, 1948, but since such chamber forms no part of the present invention, illustration thereof in this application deemed unnecessary.

'The opposite end of the cylinder II has mounted therein a converging lens ll best shown in Fig. 10, and a mirror l5 arranged substantially at an angle of to the axis of the cylinder. whereby when the cylinder is positioned horizontally the light which traverses the cylinder axial- 1y fromthe opening S in the light source holder is reflected downwardly onto the interferometric gauge indicated generally by numeral IS. The lens I4 is a convergent lens located substantially at its focal length from the light source whereby the light rays emitted by the source are brought into parallelism.

The gauge l6 comprisesa frame or base ll having a flat bottom surface slidably mounted on .a plane table or gauge platform It, (Fig. 11).

A pair of optical flats l8 and 2| of suitable material such as fused quartz are mounted on the base I] with freedom for slight pivotabie movement with respect thereto. As best shown in Figs. 3 and 4, the lower fiat 2| is mounted in a holder 22 in any suitable manner, and the holder is hinged to the base by means of flexible metal plates 23 and 24 whereby static friction of the joint is substantially eliminated. These plates are rigidly clamped to the base by means of blocks 25 and 26 and cap screws 21, 28, 29 and 3|, and are similarly connected to the holder 22 by means of clamp blocks 32 and 33, and cap screws 34, 35, 36 and 31.

In order that the hinged mounting may be made secure and without possibility of slippage, the surface of the elements adjacent to the cap screws is countersunk as shown at 38 in Fig. 6 and the clamping blocks are offset as shown at 39 in order to slightly deform the hinge adjacent the anchoring screws.

The upper optical flat i9 is similarly hinged to the base I1, and for this purpose a rigid yoke member II is tightly clamped against the side of said base as by means of a block 42 and cap screws 43. The holder 44 for theupper flat I! is connected to the ends of the yoke by means of hinge members 45 and 46 which are fixed to the yoke by clamping blocks 41, .48 and cap screws 49, 5|, 52, 53, and to the holder H by clamping blocks 54, and cap screws 56, 51, 58, and 59.

Micrometric means for adjusting the yoke 4| vertically are provided in the form of bolts 6|, 62 threaded through the ends of the yoke and into threaded bushings 63, 64. The pitches of the two threaded portions of bolts 6|, 62 are preferably slightly different so as to provide a very fine adjustment of the yoke by differential action. The bushings 63, 64 are adjustably fitted in vertical openings 5-, 66 in the base lI thereby providing a coarse preliminary adjustment of the yoke. The bushings are maintained in adjusted position by suitable means such as set screws 61 and 68. i

A leveling optical flat 69 is adjustably mounted on the base ll below the gauge flat 2|. and substantially parallel therewith. As shown in Fig. 3 this is accomplished by means of a carrier ll supported on the base by means of three adjust-- the base I! in order to locate the carrier with respect to the base. The carrier II is preferably arranged to enter loosely within a suitable conforming recess 10 in the-top surface of the base 11, as shown in Fig. 3.

The upper and lower surfaces of the lower gauge flat 2| are formed at a small dihedral angle such 'as twenty minutes, so that the beam of light from the light source I is reflected from said surfaces at slightly diiferent angles. allows observation of the interference pattern between the light reflected from the upper surface of flat 2| and lower surface of flat l9 separate from the pattern formed by the light reflected from the lower surface of flat 2| and the upper surface of the leveling flat 68. The shifting of the optical system from one pattern to the other is readily accomplished by a slight adjustment of the support III for the cylinder I.

Gauge arms I8 and 11 are rigidly attached to the carriers 44 and 22 respectively, and adjacent their free ends are provided with jewel points It and 19 arranged in vertical alignment. The arm This 1! is preferably formed integrally with the carrier 22, and is provided with a longitudinal slot 8| in which the arm 16 is freely movable. Gauge arm I8 is preferably attached to the carrier 44 in the manner indicated in Fig. 7, where the carrier is illustrated as having an undercut vertical guideway 82, and the arm 18 is provided with a dovetail extension 83 having a sliding fit in the guideway 82 whereby the vertical distance between the jewel points 18, I9 may be varied to permit measurement of a suitable range of diameters. Cap screws 84 are arranged to traverse the base 85 of the arm I6 and bear against the surface of the carrier 44 in order to immobilize the arm with respect to the carrier. Slight lateral adjustment of the arm 15 is obtained by selective adjustment of the cap screws 84 whereby the jewel gauge point 18 may be located directly above the companion gauge point 19.

Micrometric means are illustrated for adjusting the lower gauge fiat 2| with respect to the base II about its hinge connection 23, 24. As shown in Fig. 3, this means is in the form of an arm 88 projecting from the carrier 22 and resting on a plunger 81 which is vertically slidable in a hollow guide member 88 fixed to or formed as a part of the base IT. The position of the plunger 81 within the guide 89 is determined by a spring9I therein, the tension of which isadjustable by means of a threaded plug 92. This adjusting means may be omitted if desired, as shown in Fig. 11, in order to eliminate the slight amount of static friction caused thereby, and its function of balancing the weight of the lower flat 2| and its carrier, and maintaining a light pressure of the gauge point 19 on the interior of the part to be gauged, may be accomplished by merely preloading or giving a suitable set to the spring hinges 23 and 24. I

The viewing system for the interferometric gauge comprises a prism 93 mounted in the holder 8 with its edge closely adjacent the light opening 9, and a convergent lens 94 mounted in the inner end of a tube 95 suitably fixed to the cylinder II as by means of a clamp bracket 96. vAn eyepiece 91 is mounted in the outer end of a barrel 98 which is slidable in the tube 95, the

' eye piece being adjustable by .any suitable focusing means such as indicated at 99.

The optical ystem is so arranged that the light reflected fro 'the adjacent surfaces of the optical flats is converged and directed onto the prism 83, and the interference pattern caused by the light reflected from said surfaces is projected by the lens 94 in proper position to be observed by means of the eye piece 91.

The gauge here disclosed is intended more particularly to facilitate accurate comparisons of the internal dimensions of hollow parts. For this purpose, a fixture is provided for holding the part to be gauged, and the arms of the gauge are so adjusted that a predetermined interference pattern is formed by the gauge when the gauge points 18, 19 are exactly at the standard dimensional distance from each other. When the gauge is then introduced into the part to be gauged, any divergence in accuracy of the part will cause a change in the interference pattern which change can be used as an index to measure such inaccuracy.

For the purpose of conveniently calibrating and using the gauge, the plane table I8 is provided with space for carrying a standard gauging fixture I00, a mounting fixture IOI for holding parts to be gauged, and straight guide means I02, I03

adjacent said two fixtures arranged to be engaged by the flat sides of the gauge base I! to provide. for rectilinear movement thereof on the table I8 so as to move the gauge arms 16, and 11 into and out of the parts to be gauged. The standard gauging fixture I00 comprises a platform I04 on which rests an optical flat I05 of quartz or other suitable material. A second optical fiat I08 is superimposed over the flat I05, vertically spaced therefrom by standard gauge blocks I01, I08 of the Johannson type whereby the upper surface I09 of the lower flat is spaced from the lower surface III of the upper flat by a distance. which is precisely equal to the desired dimension of the part to be gauged.

The perpendicularity of the straight line joining the contact points 18 and I0 with respect to the fused quartz plates in a direction longitudinal with respect to the lever arms of the caliper may be obtained by setting the jewels over the line joining the two forward leveling screws I I2 and H4 (Fig. 11) of the standard bearing fixture and raising and lowering the third leveling point II3. If considerable motion of this point /2 turn or more in either direction) causes no change in the fringes and further motion in either direction causes a motion indicating. an increase in distance between the contact points of the jewels then the perpendicularity in this direction is suiiicientlyaccurate." "A similar method may be used to secure perpendicularity in a lateral direction.

The first step in the calibration of the gauge is the adjustment of the lower standard gauging flat I05 so as to level up the surface I09 thereof with respect to the surface of the plane table I8. This is accomplished by adjustment of three leveling screws H2, H3 and H4 which raise and lower the platform I04 with respect to its base 5. The operation consists in sliding the interferometric gauge on its base I'I along the guide I03 until the gauge arms 16, TI enter the space between the flats I05, I06 with the jewel point I9 of the lower arm 11 in engagement with the upper surface I09 of the lowerfiat I05. The leveling fiat 69 is then adjusted by means of the cap screws I2, I3 and I4 until an interference pattern is formed, by the light reflected from the lower surface of the flat 2| and the upper surface of the leveling fiat 69. Preferably the adjustment is continued until the interference pattern consists of a suitable low number of fringes,

the spacing of the fringes being recorded by counting the number of fringes between a pair of reference points a certain distance apart on a line ruled on one of the optical fiat surfaces perpendicular to the plane including the axes of the hinges of the flat holders. The interferometric gauge is then moved farther along the guide I03, the fringed pattern again observed, and the leveling screws I I2,.II3, II4 adjusted until longitudinal movement of the interferometric gauge produces no change in the interference pattern. The surface I09 is thus proved to be parallel with the surface of the plane table I8 in the direction of the guide I03. The interferometric gauge is then slid laterally on the plane table I8, and the process repeated until' lateral movement of the gauge does not affect the interference pattern. The surface I09 of the flat I05 is thenleveled up with respect to the surface of the plane table I8. I

The observing system of the gauge is then shifted slightly in order to bring into the field of view the interference pattern between the adja- 7 cent surfaces of the two gauge flats I9, 2|. This is preferably done by means of the adjusting means illustrated in Fig. 1. I As there shown, the bracket I2 is connected to the supporting cross bar I3 by a clamp member I40 fixed to the cross bar by clamping screws I4I, I42. The bracket I2 is suspended from the clamp I40 by suitable means such as a bolt I 43 loosely seated in the clamp member and so threaded into the bracket as to permit a small amount of universal movement of the bracket. The adjacent surfaces of the clamp and bracket are formed with flanges I44 and'I45 respectively, which are maintained in slightly spaced relationby the bolt I43, and adjusting screwsl46, I41 and I48 are threaded through the clamp flange I 44 so as to bear on the bracket flange I45 and provide convenient means for shifting or tilting the viewing system.

After the observing system has been thus shifted, the upper flat is adjusted by means of the differentially threaded adjusting screws BI, 52 for the yoke 4I until a suitable pattern of fringes is obtained, and the number of fringes between standard reference lines on one of the flats counted and recorded. It is then known that when the diamond points I8, I9 are separated by the standard distance determined by the height of the gauge blocks I01, I08, the recorded number of fringes are produced in the measured portion of the surfaces of the optical When a part, such as a gauge ring H6 is to be measured, it is placed in a suitable holder I I1 and held against the surface II8 thereof by suitable means such as a plate II 9 having pull rods I20 actuated by springs I2I for pressing the plate against the holder 1. The holder is seated on the upper surface of the mounting fixture IOI,

being positioned thereon by adjusting means comprising screws I22, I23, I24 and eccentric buttons I25, I20, I21 respectively. Rotation of the buttons causes the buttons to move the holder III laterally to properly locate the gauge ring IIG with respect to the gauge arms I6, TI of the interferometric gauge after which the positions of the buttons are fixed by tightening the screws I22, I23, and I24. The mounting fixture MI is provided with means I28 for adjusting the upper plate I29 of said fixture about a pivot I3I. It is also provided with means I32 for moving the entire fixture laterally on the plane table I9 and also with screws. I33, I34, I35 by which the upper plate I29 may be leveled with respect'to the surface of the plane table.

When the ring is thus mounted in the holder 1, and the latter seated on the fixture III, the interferometric gauge is withdrawn from 'the standard gauging fixture I04 and slid over on the plane table I9 against the guide I02 and advanced toward the fixture IOI until the gauge arms I0, 1'! enter the opening I35 of the ring Hi to be gauged. The fixture IN is then adjusted until the gauge point I9 is in its lowest position as determined by observing changes in the fringe pattern formed between the lower surface of the flat 2I and the upper surface of the leveling flat 59. The low point of the inside cylindrical surface of the ring is indicated by a maximum or minimum in the number of leveling fringes formed between the flats 2I and 69, further motion sideways of the ring by traversing means I32 in either direction causing the same effect on the fringe pattern. The diametrical position in the ring of the two contact points is indicated by a maximum or minimum in the number of measuring fringes formed between the flats 2| and I9, a further motion by traversing means I32 in either direction causing the same change in this fringe pattern.

At this time, the holder is raised or lowered, and leveled until the line forming the lowest part of the cylindrical surface of the ring'is brought into the same plane (with respect to the measuring device) as surface I09 of flat I05. The adiustment of said line from which the diameters are to be measured, so that it will have the same relation to the measuring device as the lower surface of the standard (surface I09 of fiat I05) is obtained by manipulating the work holder by the adjusting means I33, I34, I35 and I25, I20, I21 and I20 until the same fringe pattern is obtained for two longitudinal positions of the measuring device along the guide I02, the low point as indicated by a static condition of the levelin g fringes formed by flats 2| and 69, being chosen at both positions. i 7

When the jewel points have thus been properly positioned, the observing system is again shifted to the pattern between the gauging flats I9, 2|, and the number of fringes between the reference points thereoncounted. The difference between the number of fringes so counted and the number of fringes observed when the gauge was calibrated by means of the standard gauging fixture gives an accurate measure of the divergence of the diameter of opening I36 from the desired standard dimension.

Thegauge measurement corresponding to one light fringe will of course vary with the wave length of the light used. When a substantially monochromatic light source of known wave length is used, this relationship may be readily calculated. When, for instance, the light source is a helium lamp which emits light having a wave length of .58'75316 micron and the length of the gauge arms from hinge to contact point is 1.5 inches, this relation may be calculated as follows:

Let

L=the length of the gauge arm from the hinge to the jewel. A=the vertical movement of the jewel.

0=the angle at the hinge axis subtended by the distance A. L'=the distance between the measuring lines on the optical flat. A'=the chord of the angle 0 at radius L.

Then tan Also tan a Al/LI Assume L=1.5 inches And A=10 millionths of an inch Thentan When helium light of a wave length of .5875316 micron is used, one fringe indicates a difference in distance apart of the two plates of 11.56 millionths of an inch, therefore, the angle 0 will be indicated by 1 fringe when the length L is obtained by the formula.

L'=.00001l56/tan o =.00001156 1.5/.00001 =1.734 inches According to the presentv invention the interferometric gauge above described is used in a process of manufacturing inter-fitting parts requiring precise fits, to such close tolerances that the parts may be completely or substantially completely interchangeable. This has heretofore been extremely diflicult if not impossible since sufiiciently .accurate gauges for determining the 'dimensions of a the parts in the final stages of manufacture were not available. It. is,

true that air gauges and clectro-limit gauges are available commercially which are sufficiently sensitive to indicate differences in the order of a few millionths of an inch .but'these gauges are.

purely comparison instruments and must be calibrated against standards in order to interpret their readings. Moreover-such instruments are not adapted to compare the measurement of the inner diameter of a cylindrical surface with the perpendicular distance between two parallel planes. Such standard interplanar distances with the requisite accuracy are commercially available in the form of 'Johannson blocks but 'suificiently accurate standards are practically unobtainable directly in any-other form. .The control-of the manufacture of parts can be no more accurate than the means for calibrating the comparator gauges- The interferometricgauge here disclosed permits the calibration of such comparator gauges with extreme accuracy hitherto unobtainable by commercial gauging apparatus and methods.

The process of micrometrically accurate manufacture of articles havin internal cylindrical surfaces according to the present invention may be illustrated for example by the manufacture of cylindrical bushings of fuel injection pumps for internal combustion engines. Such bushings are now being manufactured to a tolerance of plus or minus five millionths of an inch.

The bushings are firstroughed out in any suitable manner, then the interior cylindrical surface is ground to approximate size, the size being tested by a stepped series of plug gauges until the next to the largest plug, which may for instance be three ten-thousandthsof an inch under size, enters the bushing but the last one will not enter. The following stepsare then taken, first: calibration of a ring gauge to an accuracy of about plus or minus 3 or 4 millionths of an inch'by the above described method of the use of the interferometric gaugesecond: the use of such calibrated ring gauge to calibrate a comparator gauge such as an air gauge or an electrolimit gauge, and third: the use of the comparator gauge to measure the interior diameter of the bushing being manufactured during the final operation of grinding to size. The interferometric gauge may also be used for inspection of the bushing directly, as a final check on the whole process.

The process above outlined enables the accurate control of the final manufacturing operations with uniformity and convenience since the ring gauges may be used for a considerable length of time with periodic checking on the interferometric gauge and the latter may there fore be used to control a large number o[ inspection operations. Since the accuracy of the interferometric gauge is dependent only upon the accuracy of the Johannson blocks and such blocks may be obtained which are highly accurate and uniform. it is clear that this method is capable of producing parts-with the required dimensional accuracy, when suitable means are used for removing the amount of material indicatedto be necessary ,by the control gauges.

Although, certainstructure has been shown and described in detail and certain steps have been disclosedfor the utilization of such structure, it will be understood that changes may be made in the precise design and arrangement of the parts and certain steps may be omitted or added without departing from the spirit ofthe invention as defined in the claims appended hereto.

What is claimed is:

.1. In an interferometric gauge, a base, a pair of optical flats superimposed one on the other in spaced relation with their plane faces in approximate parallelism and hinged to the base o'n'an axis substantially parallel to said. plane faces,

means for directing a beam parallel light rays on said flats substantially normal thereto, the fiat nearest the light-directing means being light,- transmitting, means for observing the interfereence pattern in the light reflected from the flats caused by a divergency from exact. parallelism thereof, a pair of caliper arms having contact points adjacent the ends thereof, means for rigidly attaching one of said arms to each flat,

said attaching means including means for adof a bar having a longitudinal slot, and the other arm is in the form of a beam lying loosely in said slot. 1

3. An interferometric gauge as set forth in claim 1 in which the attaching means for one of said arms is arranged to provide for bodily adjustment of the arm in the direction of the line joining the contact points. to permit measurement of various sizes of work while maintaining the optical flats within the desired proximity to parallelism.

4. An interferometric gauge as set forth in claim 1 including further a hinged holder for one of said flats to which the corresponding caliper arm is attached, said holder and arm having an undercut tongue and groove slidable connection, and a plurality of screw means on both sides of said connection for tightening the connection, said means being individually adjustable in order to set the position of the contact point carried by the arm.

5. In an interferometric internal gauge, a base, an optical fiat, a holder therefore, means for hingedly mounting the holder on the base, a second optical flat and a holder therefore, at least one of said optical flats being light-transmitting; and means for hingedly mounting the second holder on the base with its pivotal axis parallel and coplanar with the axis of the first, holder, said second mounting means comprising a yoke member. having a hinged connection at its ends to the second optical flat holder and means for rigidly mounting the yoke members on the base.

6. An interferometric gauge as set forth in claim 5 including further a pair of caliper arms rigidly fixed to said holders and extending substantially normal to the plane of said hinge axes.

7. An interferometric internal gauge as set forth in claim 5 including further, micrometric means for adjusting the ilrst holder about its axis, and in which the means for mounting the yoke member in the base includes means providing both a coarse anda fine adjustment of said yoke with respect to the base.

8. An interferometric internal" gauge as set forth in claim 5 in which said yoke is arranged to straddle the hinged mounting means for the first holder. 1

9. An interferometric internal gauge asset forth in claim 5 in which the mounting means for the yoke includes means for clamping the central portion of the yoke to the base and threaded means for independently adjusting the spacing of the ends of the yoke from the base.

10. An interferometric internal gauge as set forth in claim 5 in which said'hinged mounting means comprise flexible plates and means for anchoring said plates including studs traversing said plates and threaded into the parts to be connected, and means for slightly deforming the portions of the plates surrounding the studs to prevent slippage. Y v

llrAn interferometric internal gauge as set forth-in claim 5 in which said hinged mounting means comprise flexible plates and means for anchoring said plates including studs traversing the plates and threaded into openings in the parts to be connected, said openings rbeing countersunk and said studs having heads conforming to the countersinks and arranged to press the plates into the countersinks.

12. In an interferometric gauge a horizontal fiat table, gauge means including a base having a flat bottom surface slidably mounted on the table, a pair of gau ing optical flats superimposed one on the other in spaced substantially parallel relation and hingedto the base on parallel horizontal axes, at leastthe upper one of said optical flats being light-transmitting, a caliper arm flxed to each flat, said arms having contact points located in a line perpendicular to the table.

13. In an interferometric gauge a horizontal plane table. a rectilinear guide thereon, gauge means including a base having a flat bottom surface slidably mounted on the table and a plane lateral surface for engagement with the guide, a

pair of gauging optical flats superimposed one on the other in spaced substantially parallel relation and hinged to the base on horizontal axes lyingin, the same vertical plane, both said optical flats being light-transmitting, a caliper arm ilxed'to each flat. said. arms having contact points located in a line perpendicular to the table and a. leveling optical ilat adjustably mounted on the base beneath and substantially parallel to the saueins fl ts- REFERENCES CITED The following references are of record in the tile of this patent:

OTHER REFERENCES Publication Bureau of Standards Scientific. copy in'8814I (Div. 7); Paper #365, 1920, also 33-14311 (Div. 62) pp. 669678 and Plate facing 674, "A New Interferential Dilatometer, by

Priest. I 

